Project Inferno is designed to drill down through the Earth's core, where
it will release a powerful new energy source called Stahlman's Gas, named
after the project's director. But the Doctor realises that unleashing
Stahlman's Gas will have horrible consequences for the planet, and indeed
his fears are confirmed when a substance oozing up from the drill shaft
begins mutating men into bestial Primords. Before the Doctor can do
anything to stop Stahlman, however, a power surge in the TARDIS console
sends him to a hostile parallel universe where Project Inferno is nearing
completion.

Production

The story originally intended to cap off Doctor Who's seventh
season was “The Mists Of Madness” from Brian Wright, which was
commissioned by script editor Terrance Dicks on February 17th, 1969. Some
months later, however, Wright accepted an academic writing post in
Bristol, leaving him no time to complete his Doctor Who assignment.
Season Seven's final slot was consequently still empty when Barry Letts
became the programme's producer in October.

Around this time, Dicks considered two storylines as prospective season
finales. One was “The Shadow People” by husband-and-wife team
Charlotte and Dennis Plimmer; the Plimmers submitted this to the Doctor
Who production office on November 10th. Shortly thereafter, however,
“The Shadow People” was abandoned following a pay dispute
between the Plimmers and the BBC. This left the way open for Dicks' other
option, “The Mo-Hole Project” by Don Houghton.

Don Houghton was inspired by a plan to drill through the
boundary between Earth's mantle and crust

Houghton had been a script editor on the soap opera Crossroads when
Dicks had written for the programme, and had also worked with Letts on
Emergency Ward 10. For Doctor Who, Houghton found
inspiration in a scientific article he had read about five years earlier,
concerning an American proposal to drill more than five kilometres through
the Earth's mantle in the Pacific Ocean to the Mohorovicic discontinuity,
the boundary between the planet's mantle and its crust. Since Houghton's
story would have to carry seven episodes, Letts and Dicks suggested a
subplot involving the Doctor being cast into a parallel universe. A
storyline for “The Mo-Hole Project” -- taking its name from
the American mission -- was requested on November 27th.

The title of Serial DDD had become “Operation: Mole-Bore” by
the time Houghton delivered it in early December, and simply “The
Mole-Bore” around the time full scripts were commissioned on January
9th, 1970 (although Houghton had already started writing them by this
time). A major change requested by Letts and Dicks was the addition of
some sort of monster to the proceedings, resulting in the inclusion of the
Primords (initially spelt “Primeords”). As well, Sir Keith
Mulvaney become Sir Keith Gold (after the surname Rose was also
considered), while Private Peters' last name was changed to Wyatt.
Houghton was also asked to introduce a type of alien karate for the Doctor
to use in hand-to-hand combat; this was originally “Feltian”
karate before being renamed “Venusian” karate.

The director assigned to “The Mole-Bore” was Douglas Camfield,
whose last Doctor Who work had been on The
Invasion a year earlier. It was Camfield's decision to have the
Primords appear as werewolves rather than ape-creatures, as Letts had
conceived them. In the role of Petra Williams, Camfield cast his wife,
Sheila Dunn, after Hammer horror actress Kate O'Mara proved
unavailable.

Camfield also hired John Levene to play a UNIT sergeant who had gone
unnamed in Houghton's scripts. Levene had played UNIT Corporal Benton in
The Invasion and so it was decided that this
should be the same character, having been promoted between appearances.
Levene was contracted on February 23rd, and continuity was further
maintained through the character's last-minute inclusion in The Ambassadors Of Death, the previous serial
in production. Around the same time, Serial DDD was retitled again, this
time to “Project Inferno”.

Concerned that Doctor Who
would be cancelled, Barry Letts and Terrance Dicks had both been
pursuing replacement projects

In late February, as “Project Inferno” neared the start of
filming, the production office was informed by Head of Serials Ronnie
Marsh that the new format for Doctor Who engineered by Letts'
predecessors, Peter Bryant and Derrick Sherwin, had been successful enough
to guarantee the show an eighth season. Letts and Dicks had been
sufficiently concerned about the possibility of cancellation that each had
been pursuing replacement projects -- entitled Snowy White and
Better Late, respectively. Jon Pertwee's services were secured for
another twenty-five episodes on March 9th, with Nicholas Courtney
committing to the new season on April 8th.

However, Letts was not happy with the Liz Shaw character, feeling she was
too sophisticated to serve as nothing more than the Doctor's sidekick. The
decision was therefore made to replace Liz with a new companion for Season
Eight; as it transpired, Caroline John was now pregnant and likely would
not have been available for the eighth recording block anyway.

Meanwhile, location filming for “Project Inferno” had begun on
March 31st. The lone venue used for the serial was Berry Wiggins & Co, an
oil refinery situated near Strood in Kent which served as the Inferno
drill site in both the regular and parallel universes (referred to as
“Warp I” and “Warp II” in Houghton's scripts).
Work continued until April 3rd. On this last day, an accident occurred
when Pertwee, driving Bessie, hit stuntman Alan Chuntz while recording the
Warp II scene where the Doctor tries to avoid the parallel-world soldiers.
Despite needing eighteen stitches, Chuntz toughed it out and returned to
work in order to lift Pertwee's spirits. Nonetheless, the delay meant that
some episode one scenes -- of Benton and other UNIT troops searching for
Slocum, and of the Primord murdering a UNIT soldier named Collins -- had
to be abandoned.

Three days were then spent at the Ealing Television Film Studios beginning
on April 6th. Of particular concern during these sessions were the
Doctor's journey between universes (through what the scripts called the
“Nightmare Warp”) as well as various drill head sequences,
including the transformations of both Platoon Under Leader Benton and
Professor Stahlman into Primords. Around this time, the serial's title was
truncated to simply Inferno.

Barry Letts experimented with the recording schedule, with
two episodes taped over two consecutive days on a biweekly basis

As he had done to a more limited extent on The
Silurians earlier in the season, Letts decided to experiment with
the recording schedule for Inferno. Normally, there was one
recording day for Doctor Who each week, during which one episode
would be taped. However, Letts thought it would be more efficient to
record two episodes over two consecutive days on a biweekly basis. This
way, sets could be left up overnight, reducing the wear and tear which
came from having them taken down and set back up every week. As such,
Inferno would be made in four distinct recording sessions rather
than seven.

The first such session covered Thursday, April 23rd and Friday the 24th
and was devoted to episodes one and two. However, because so much of these
installments had been completed on film, Camfield elected to use the first
day entirely for camera rehearsals, confining recording only to the
Friday. Unfortunately, things did not go well on this day. Pertwee would
not follow Camfield's directions and when the star argued with production
assistant Chris D'Oyly John, an irate Camfield descended from the
production gallery, forcing Dunn to intervene and persuade Pertwee to
comply.

Things would come to a head during rehearsals on episode three at the
start of the following week. By this time, John was also unhappy because
Courtney had convinced Camfield and Letts that it should be the Brigade
Leader who interrogates the Doctor in part three rather than Section
Leader Shaw. As tensions mounted on the 27th, Camfield collapsed during
rehearsals, forcing D'Oyly John to summon Letts. Dunn revealed that her
husband was suffering from a heart murmur, and it was agreed that Camfield
should be removed from Inferno to safeguard his health. Letts'
first inclination was to hire a new director to finish the story but,
because Camfield had already completed much of the planning for the final
five episodes, Letts decided to take over the story himself. He had
previously helmed The Enemy Of The World two
years earlier.

With Letts now in charge, recording resumed on May 7th and 8th, which were
again a Thursday and Friday; work on these days concentrated on episodes
three and four, as well as some material from part six. Exactly two weeks
later, episode five was taped alongside the remainder of the sixth
installment over May 21st and 22nd. Unlike the earlier recording days,
which had taken place in BBC Television Centre Studio 3, work on these
dates shifted to TC6 instead. Pertwee had a rare opportunity to play a
different role during this session, as he prerecorded the announcer's
voice (performed in imitation of Nazi propagandist William Joyce) heard
over the Brigade Leader's radio. Cast and crew then returned to TC3 for
the final episode, which was taped on Friday, May 29th, completing both
the serial and the recording block.

A scene where Jon Pertwee provided the voice of the radio
announcer was removed due to concerns that he was too recognisable

In post-production, Letts became concerned that Pertwee's voice was too
recognisable as the radio announcer, and removed this segment from episode
five; nonetheless, it would be retained on the prints sold
internationally. The broadcast of Inferno part seven on June 20th
brought Doctor Who's seventh season to a close.

This also marked the end of John's regular involvement in the programme
and the start of a lengthy hiatus in her acting career while she raised
her family with husband Geoffrey Beevers. John returned to the profession
in the early Eighties with an appearance in the Letts-produced version of
The Hound Of The Baskervilles, starring Fourth Doctor Tom Baker as
Sherlock Holmes. She also reprised her role as Liz on several occasions --
on TV in both The Five Doctors and Dimensions In Time, and in the PROBE
direct-to-video series. She made her debut with Big Finish Productions
playing Madam Salvador in the Seventh Doctor audio drama Dust
Breeding, before making several appearances as Liz for the company
in their Companion Chronicles range, beginning with The Blue
Tooth. John continued to appear in a variety of television and movie
projects, such as the feature film Love Actually, prior to her
death from cancer on June 5th, 2012.

Sources

Doctor Who: The Handbook: The Third Doctor by David J Howe and
Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.